Merge 'bindings/javascript: Add source property to Statement' from Anton Harniakou

Let's you get the source string that was used to create the prepared
statement.

Reviewed-by: Diego Reis (@el-yawd)

Closes #1670

bindings/javascript/wrapper.js

@@ -178,10 +178,18 @@ class Statement {
     return this;
   }
 
+  get source() {
+    return this.stmt.source;
+  }
+
   get reader() {
     throw new Error("not implemented");
   }
 
+  get source() {
+    return this.stmt.source;
+  }
+
   get database() {
     return this.db;
   }